1. Field of the Invention
This invention relates to aviation turbo oils having high load carrying capacity, said oil comprising a base oil and additives which impart the load carrying capacity.
2. Description of the Related Art
Lubricants must possess a high load carrying capacity in order to be able to transmit strong forces between mating metal surfaces, gears for example, while controlling (preventing or minimizing) metal damage and wear under heavily loaded conditions. Extreme Pressure (EP) additives present in the lubricant operate to reduce and minimize metal damage by preventing seizure and welding between metal surfaces working under extreme pressure conditions. Under such conditions (i.e., boundary lubrication) the ability of the lubricant to prevent wear is no longer dependent on the hydrodynamic (i.e., viscometric) properties of the lubricant but on its chemical (EP) properties.
EP additives function by reacting chemically with the metal surfaces producing a sacrificial layer of low shear strength thereby minimizing wear of metal surfaces and preventing welding (seizure) of the moving, interfacing metal parts.
EP additives usually consist of sulfur, phosphorus or chlorine containing compounds. These atoms are the reactive centers of the EP additives, and consequently can also be quite corrosive to the metals they are intended to protect.
EP additives must meet a difficult combination of requirements. It must possess high surface activity in order to attain complete surface coverage over the entire rubbing surfaces which are in contact. The EP additive must be sufficiently surface active to successfully compete for reactive surface sites of the metal with other components present in the oil (e.g., the base stock itself, corrosion inhibitor, etc.) yet at a sufficiently low concentration in order to minimize adverse interactions with the other components in the lubricating oil.
Extensive surface coverage however, is in itself an insufficient condition for an EP additive's activity. The additive should react with the metal surfaces only under high load conditions when high flash temperatures are attained in the contact region, that is when there is the abrupt transition from boundary lubrication conditions (which are satisfied by the antiwear properties of the oil) to EP conditions (which rely on the chemical interaction of the EP additive with the metal). The ideal EP additive will react with the metal surfaces under the extreme conditions of pressure and temperature of the mating surfaces and not before these conditions are attained. Premature reaction of the EP additive with the metal results in significant corrosion.
Widely used EP additives are sulfurized fatty oils, sulfur chloride treated fatty oils, chlorinated paraffin wax, chlorinated paraffin wax sulfides, aliphatic and aromatic disulfides such as dibenzyldisulfide, dibutyl disulfide, chlorobenzyl disulfide. Chlorine containing EP additives are not suitable for use in aviation turbine oils due to their corrosivity, as are most sulfur containing EP additives. EP additives for aviation turbine oils must also be ashless, so EP additives such as lead naphthenates are unsuitable.
Aviation turbo oils typically have employed anti wear/extreme pressure additives including hydrocarbyl phosphate esters, particularly trihydrocarbyl phosphate esters in which the hydrocarbyl radical is an aryl or alkaryl radical or mixture thereof. Particular anti wear/extreme pressure additives which have been used include tricresyl phosphate, triaryl phosphate and mixtures thereof.
Other extreme pressure additives include those having sulfhydril (e.g., mercapto groups) but in general they have been found to be corrosive to copper.
It would be beneficial if an additive could be identified which imparted load carrying capability to the oil at low treat rates and which was noncorrosive to copper and compatible with the other materials used in the engine and seals.